During the transfer molding or injection molding process, the epoxy is exposed to elevated temperatures and pressure. At temperature and under pressure, the epoxy mold compound’s viscosity will drop significantly going from a solid at room temperature to a liquid when exposed to temperatures of 160°C to 200°C under 1-2 bars of pressure. Unlike a thermoplastic however, epoxy molding compounds are thermoset plastics. This means that at continuous exposure to temperature, the viscosity will first drop to its minimum achievable viscosity and then rise again as the epoxy cures until the epoxy stops flowing completely. (See Figure 1).
This is an important step in the molding process as it allows the epoxy to flow into the mold cavities and encapsulate the device or component before curing to provide strong mechanical protection. Failure to completely encapsulate the part is called “incomplete fill”. The minimum achievable viscosity can thus be important to ensure that the parts are completely encapsulated and that there are no cases of incomplete fill.
An industry standard test to measure the viscosity is called a Shimadzu viscosity flow test. This test measures the viscosity of an epoxy molding compound at a constant temperature, typically using the epoxy molding temperature of 175°C. The curve in Figure 2 shows how the minimum achievable viscosity is affected after exposure to temperature for extended periods of time.
The difference in minimum achievable viscosity is different depending on the epoxy compound itself and the time and temperature that the epoxy was stored. Measuring this flow at different times and temperatures will therefore give you the extent of this property change.
Please stay tuned for the next part in this series to explore the behaviour of epoxy molding compounds.
Please visit us at www.caplinq.com to learn more about our whole range of molding compounds including our semiconductor epoxy molding compounds, industrial-grade fiberglass-reinforced epoxy molding compounds, and optically clear epoxy molding compounds (CMC) for optoelectronics. If you have any other questions about the effects of time and temperature on the cure of epoxy molding compounds please feel free to leave a comment below or don’t hesitate to contact us.
Recently, a customer complained that he had a hard time correlating the wire bond sweep with material properties like spiral flow, filler content, filler size, etc. He was using mold flow analysis to predict wire sweep, but the results didn’t seem to make sense to him.
The reason for this is because these properties can be mutually exclusive, and combinations of these properties and result in vastly different properties.
Let me start with a basic example to make my point. Imagine you have a mold compound that is 80% filled by weight with spherical silica with an average particle size of 100um. If we did nothing else but swap out this filler a silica nanoparticle with an average particle size of 0.1um, what would be expect in material property differences?
Though both particle sizes may predict similar wire sweep, in reality, the nanoparticle-filled mold compound would be so thick that it wouldn’t be able to flow at all. Epoxy mold compound formulations often have multiple different sizes (from nanoparticles to several hundred microns) and shapes of filler including spherical, angular and other. These combinations are done exactly to optimize packing density, viscosity and flow.
Likewise, spiral flow itself is not a measure of viscosity. It is a measure of, well… flow. Spiral flow measures the distance a mold compound will flow when exposed to mold temperature and pressure. All other things being equal, a material with a lower viscosity will flow further than one with a higher viscosity. But all things are rarely equal. If the low viscosity material has a shorter gel time, than it may flow further while it is still liquid, but will stop flowing sooner than the material with the longer gel time.
Epoxy molding compound formulation is certainly as much of an art as a science. So what is one to do
All other things being equal, if the